To identify primary and secondary sources
A primary source is anything that was created at the time of the event/time period that is being studied, or made afterwards by someone who was there.
Examples include:
Newspapers
Artifacts
Letters/correspondence
Secondary sources are usually created by people who have analysed primary sources. They might be historians or people who are passionate about an event/era. To be reliable it would ideally be created by someone who has specialised skills or knowledge in that area.
Examples include:
Articles
Biographies
Textbooks
Both can be really useful for research provided we check the reliability- for example: letters written by a child might exaggerate or not be able to communicate what is happening in a clear way. This doesn't make it a useless source, but we have to be aware of these sorts of influences on the what we are reading.